Determination of nitrogen



Aug. 12, 1 969 R. L. MARTIN ET AL 3,4

DETERMINATION OF NITROGEN Filed Feb. 21, 1966 Heating Gail Ronald L. Martin Nitrogen Compound Humid/fled Hydrogen INVENTORS.

Robert J. Fla/mar United States Patent DETERMINATION OF NITROGEN Ronald L. Martin, 'Glenwood, and Robert J. Flannery,

Olympia Fields, 111., assignors to Standard Oil Company, Chicago, 11]., a corporation of Indiana Filed Feb. 21, 1966, Ser. No. 528,732 Int. Cl. B01k N00 US. Cl. 204-1 4 Claims ABSTRACT OF THE DISCLOSURE An apparatus and process for determining the nitrogen content of a nitrogen-containing material by catalytically converting the nitrogen to ammonia and by automatically titrating the ammonia electrochemically, in the presence of hydrogen gas, with hydrogen ions which are generated according to the equation:

This invention relates to a method and apparatus for automatic quantitative analysis. More specifically to a process and apparatus for the detection of the nitrogen content of nitrogen-containing materials. Analyses for nitrogen are required in many research and process laboratories. In the petroleum industry, for example, the several deleterious efiects of nitrogen compounds make it necessary to determine nitrogen in concentrations down to about 1 p.p.m. in a variety of feeds and products.

Essentially all of the methods for determining total nitrogen use one of three basic approaches, Kjeldahl, Dumas, or ter Meulen. Modifications and improvements of the basic methods are many, yet none have both high sensitivity and short analysis times. Kjeldahl methods, which employ a sulfuric acid digestion and steam distillation of ammonia, are the most popular and are relatively sensitive, but require a careful and experienced analysis and take at least an hour. Dumas methods, which use an oxidation of nitrogen compounds to elemental nitrogen over hot copper oxide, require less time but are less sensi' tive. The third method, the ter Meulen method, has heretofore not been extensively used nor has it been employed along with an automatic tritrating system. This method employs the catalytic production of ammonia. A process has now been discovered which offers the advantages of high sensitivity and greatly reduced analysis time. For example, this process and apparatus can be used to determine nitrogen in quantities smaller than 1 p.p.m. in less than minutes. In addition, this process and apparatus can be used with a gas chromatography system for the selective detection of nitrogen compounds.

The accompanying drawing illustrates a particular embodiment of this invention.

The figure is a schematic illustration of the nitrogen analysis apparatus.

Briefly stated, the process comprises converting nitrogen contained in the material to be analyzed to ammonia, said nitrogen being catalytically convertible to ammonia, and automatically titrating the ammonia electrochemically. More specifically, the nitrogen compounds are passed over a catalyst, nickel-on-magnesium oxide for example, in a stream of nitrogen, thereby converting the nitrogen contained in the compounds to ammonia. The ammonia is then titrated automatically in a micro titration cell. This titration may be carried out coulometrically with any known ammonia titratable reactant. For example, as disclosed in a copending application, Serial No. 529,134, filed even date with the instant application, coulometrically generated hydrogen ions provide excellent results. Ammonia entering the titration cell is titrated automatically 3,461,042 Patented Aug. 12, 1969 with hydrogen ions generated coulometrically within the cell. The titration reaction can be written as:

NH OH+H+- NH ++H O The reaction for coulometric generation of titrant is:

Current used in the generation is amplified and recorded against time to give a direct measure of the amount of ammonia titrated.

A specific embodiment of the invention is as follows:

The nickel-on-magnesium catalyst is prepared by adding a nickel nitrate hexahydrate solution slowly to a magnesium oxide slurry with vigorous stirring. The precipitate is then decanted, washed, compacted, and dried. The nickel oxide is then reduced to nickel metal by heating in the presence of hydrogen. A catalyst tube containing a catalyst bed is heated preferably to a temperature of about 440 or l0 C. The catalyst tube is then filled with a 3" section of catalyst and purged with hydrogen for thirty seconds. The catalyst tube is then connected to a titration cell, provided with an automatic stirrer and containing a suitable electrolyte.

The combination of higher temperature and humidified hydrogen permits analysis of higher boiling samples than were previously possible by catalytic means. At the lower temperatures, many or most nitrogen compounds boiling above the kerosene range failed to yield ammonia quantitatively. The major difficulty associated with the higher boiling samples is coke formation on the catalyst. This difiiculty is lessened in two ways with humidified hydrogen, the rate of coke formation is decreased, and more significantly, ammonia adsorption on the coke is decreased. Without humidified hydrogen, ammonia adsorption increased as coke accumulates and the peaks soon become too tailing to be measured accurately. A temperature of 440 C. is about optimum; higher temperatures give less adsorption of ammonia, but also lead to a more rapid accumulation of coke.

Hydrogen flow rate through the catalyst tube is not particularly critical. Quantitative ammonia production has been obtained at rates from 10 to 1,000 ml. per minute. The rate chosen for usual operationabout 500 ml. per minute-is fast enough to keep peak Widths relatively narrow and yet not so fast as to cause significant noise in the titration cell.

Briefly, the apparatus of this invention comprises a titrating zone containing an electrolyte; means communicating with the titrating zone for catalytically converting the nitrogen to ammonia; means electrically in communication with the titrating zone for determining a potential difference in the electrolyte caused by introducing the ammonia into the titrating zone; means electrically in communication with the titrating zone for continuously r generating titrant to eliminate the potential difference; and

means for measuring the amount of titrant generated, this amount being a measure of the nitrogen content.

More particularly, with reference to the figure, the apparatus contains a titration cell 4 containing an electrolyte wherein the titration reaction is carried out. In electrolytical contact with the electrolyte is a sensing electrode pair consisting of reference electrode 12 and sensor electrode 14 and a generating electrode pair consisting of anode 8 and cathode 10. Electrodes 8, 12 and 14 are positioned in a titrating zone of the cell with electrode 10 being positioned in that part of the cell which is outside the titrating zone. Mixing or difussion of electrolyte between the titrating zone and the part containing electrode 10 is prevented by glass frit 6. Electrodes 8 and 10 are interchangeable so that for a particular titration cathode 10 may be positioned within the titration zone and anode 8 outside of that zone. In addition it may be desirable for a particular reference electrode to place electrode 12 outside the titrating zone with appropriate electrolytic connection.

The material to be analyzed is placed over a catalyst bed contained within catalyst tube 2. The nitrogen compounds are converted to ammonia in catalyst tube 2 in the presence of humidified hydrogen. The gaseous ammonia is introduced into the titrating zone of cell 4. Titrant concentration is sensed by electrode pair 12 and 14. Reference electrode 12 may be any standard half cell, such as silver sulfate, lead sulfate, calomel, mercury sulfate or oxide, silver chloride, hydrogen, glass, etc.

The original concentration of titrant within the cell is fixed by a reference potential established within the electronic system. This reference potential is called a bias potential. The bias potential is applied by the control electronics to one input of the coulometer differential amplifier and is equal and opposite to the potential of the reference/sensor electrode pair, 12 and 14 which is applied to the other input. When the two voltages are equal, the amplifier receives a zero signal and titrating ions are not generated; however, when ammonia enters, a potential difference in the electrolyte is established and the voltage shift of the sensor electrode is applied to the amplifier, and a flow of current is produced in the generator circuit to form titrating ions at the anode. This generation continues until the original titrating ion concentration is restored and the reference/ sensor voltage again equals the bias voltage. The amount of titrant generated is measured and recorded by the readout, such amount being a measure of the nitrogen content in the sample. If hydrogen ions generated by the generator electrode are the titrant, which is disclosed in the copending application referred to above, the cell operates efficiently (i.e., the titrant generation peaks are the most narrow without significant overshoot) when the Original concentration of hydrogen ion titrant is kept between about and 10* moles per liter; the preferred range is 10 to 10*.

The bias setting is chosen to give hydrogen ion concentrations in the range indicated above. For example, the bias voltage, which is equal and opposite to the refe'rence/ sensor voltage (neglecting overvoltage), can be computed approximately by:

where (H+) is the desired hydrogen ion concentration and E is the standard potential of the reference electrode, E is the standard potential of the hydrogen electrode. If generation of titrant is too fast or too slow, the bias can be adjusted a few millivolts either way until it becomes optimum.

The process of this invention may also be used with a gas chromatographic column. The column selected should give efiicient separations and not react with or strongly adsorb petroleum nitrogen compounds and be able to be taken to relatively high temperatures without causing detector response or catalyst deactivation. A column satisfying these requirements was a foot by A; inch I.D. stainless steel section packed with 9% by weight of 12,000 nolecular weight polyethylene (Bakelite DYLT) on Chromosorb-W previously coated with 3% potassium cardonate. This column gave good separating efiiciencies, was unreactive, and was usable to 330 C., which was mfiicient to elute samples as high boiling as gas oils.

It is a significant feature of the titrating means of this nvention that hydrogen gas, which is used as the eluting gas if gas chromatographic separation is carried out, and vhich is necessary in the step of converting nitrogen-conaining compounds to ammonia, is also necessary in the itration step. The hydrogen gas bubbles through the reacion zone at all times and acts as a carrier for the am nonia sample. As hydrogen gas bubbles past the generator lectrode, some of the hydrogen is adsorbed thereon. It s well known that hydrogen bubbled into aqueous solution containing a platinized platinum electrode will adsorb on the electrode surface and fix its potential at a value dependent on the pH. The presence of hydrogen gas on the electrode fixes the electrode potential at a value such that hydrogen ions can be generated from hydrogen gas according to the reaction:

The following examples are given by way of illustration:

EXAMPLE I Using the procedure as outlined above, the response of the detection system was first tested by measuring the number of coulombs used in titrant generation for injection of 1.00 ,ul. of .00610 N ammonium hydroxide, which was added to the coulometric titration cell by calibrated syringe. The coulombs used were found to be .000584. Assuming 96,500 coulombs would produce 1 equivalent of H+, which would titrate 1 equivalent of NH OH or 35.0 grams, .000584 coulomb would titrate .212 g. NH OH. Actual ,ug. NH OH in 1 ,ul. of .00610 N NH OH is .214 ,ug. NH4OH.

It may, therefore, be seen that the process of this invention may be used to obtain results which are within 1% of the actual values.

EXAMPLE II Similar measurements were then made for pyridine, and the coulombs used in titrant generation were again found to be quantitative within experimental error.

Typical total nitrogen results are shown in Table I for 8 single compound synthetics and 5 petroleum fractions analyzed previously by a Kjeldahl method. The three pyridine analyses show that a wide range of concentrations can be analyzed without difficulty. The agreement for the several different types of nitrogen compounds suggests that essentially all types of organic nitrogen compounds can be analyzed accurately.

The last two analyses in Table I show that samples in the vicinity of 1 p.p.m. can be analyzed accurately without a prior concentration step.

TABLE I.ANALYSES FOR TOTAL NITROGEN Nitro en .m. Single-compound synthetics 1 g p p Known Found Pyridine 4. 2 4. 4

Phenazine 154 158 Other samples:

Light catalytic cycle oil 241 233 Shale naphtha 1, 900 1, 920

Heavy virgin gas 0 605 600 Catalytic naphtha. 2 0.5 0.8

Percolated heavy cycle oil 2 2 1. 7

1 Prepared in nitrogen-tree reformatos. 2 K eldahl analysis after a prior concentration step; estimated uncertainty is about i50 0 relative.

interferences to the nitrogen detection system are extremely few. Basic compounds other than ammonia could interfere, but few, if any such compounds, would be present or would be formed during passage through the catalyst.

Having described the invention, what is claimed is:

1. A process for continuously and rapidly determining the nitrogen content of a nitrogen-containing material, said nitrogen being catalytically convertible to ammonia, said process comprising the steps of converting said nitrogen, contained in said material, to ammonia, and automatically titrating said ammonia electrochemically, in the presence of hydrogen gas, with hydrogen ions as the titrant, said hydrogen ions obtained from said hydrogen gas.

2. The process of claim 1 wherein said nitrogen is catalytically converted to ammonia by passing said nitrogen-containing material over a nickel-on-magnesium oxide catalyst in the presence of hydrogen.

3. The process of claim 1 wherein said automatic titration is performed coulometrically.

4. A process for the determination of the nitrogen content of a nitrogen-containing material, said nitrogen being catalytically convertible to ammonia, said process comprising the steps of catalytically converting, in the presence of hydrogen, said nitrogen contained in said material to ammonia and automatically titrating said ammonia, in the presence of hydrogen gas, with coulometrically generated hydrogen ions as the titrant, said hydrogen ions obtained from said hydrogen gas according to the reaction: /2H +H++e-.

References Cited UNITED STATES PATENTS 2,758,079 8/1956 Eckfeldt 204-195 6 2,832,734 4/ 1958 Eckfeldt 204-495 2,954,336 9/1960 Grutsch 204195 OTHER REFERENCES Holowchak, Analytical Chem., vol. 24, #11, Novem 5 her 1952, pp. 17544757.

T. TUNG, Assistant Examiner US. Cl. X.R.

53 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, |-6LD +2 Dated August 3.2, 1969 Inventor(e) Ronald L. Martin et :11.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 0+, the first appearance of the word "nitrogen" should read --hyd.rogen--@ Column line 20, the word "coulomb" should be --coulombs--.

SIGNED AN'D SEALED APR 1 4.1970

(SEAL) Attest:

Edward M. Fletcher, 11'. WILLIAM E. 'SQHUYLER, JR- Gommissioner of Patents Attesting Officer 

